1. Field of the Invention
This invention generally relates to a ladder-type cable bed. This invention more particularly relates to a ladder-type cable bed incorporating roller rungs, which utilizes cable support blocks to be mounted on support rungs after an initial cable laying operation. This invention also relates to a cable laying method using such a cable bed.
2. Prior Art
Japanese Laid-Open Utility Model Publication 62-172210 discloses a ladder-type cable bed 1 incorporating & plurality of roller rungs 10 as shown in FIGS. 12 and 13, FIG. 12 being a partial perspective view and FIG. 13 being a partial front view with a longitudinally extending side wall 2 sectioned.
Each roller rung 10 comprises a plurality of disk rollers 12 turnably mounted on an axial shaft 11 which has a projection 11a (with a threaded hole) on each end thereof. Each projection 11a is fixedly connected to a corresponding side wall 2 with a threaded bolt 6 screwed through a hole provided on the side wall 2 from outside into the threaded hole of the projection 11a. A cable W (shown in phantom) generally touches two disk rollers 12 of each roller rung 10 (which turn on the axial shaft 11) when drawn onto the cable bed 1, and that portion of the cable W is supported by the two disk rollers 12 as shown in FIG. 13.
The cable bed 1 includes a plurality of such roller rungs 10 mounted in parallel at intervals across two parallel side walls 1. The cable bed 1 also includes a plurality of support rungs 3 mounted in parallel at intervals across the side walls 2 among the roller rungs 10. In general, more support rungs 3 are incorporated in a cable bed 1 than roller rungs 10 as shown in FIG. 14 partly because roller rungs 10 are much more costly to manufacture and much more fragile than support rungs 3.
In general, a cable or pipe to be drawn onto a (ladder-like) cable bed is heavy and long, and generates (during a cable laying or drawing operation) considerable friction with the cable bed or its rungs. The roller rungs 10 are expected to assist in drawing a cable W onto the cable bed 1 while the support rungs 3 support a much larger part of the cable weight in an attempt to reduce cable traction force.
Since the support rungs 3 are provided on the cable bed 1 at substantially the same level with the roller rungs 10 as shown in FIG. 14 in schematic partial side view, the friction generated between the cable W and the support rungs 3 during a cable laying operation can be so large that the cable drawing operation will be substantially hindered, since conventionally the cable W must be and is mostly supported by the support rungs 3.
It is advantageous, therefore, that a cable W does not contact support rungs 3 while initially drawn onto the cable bed 1 but contacts only the roller rungs 10.
Such an arrangement may be provided by installing on side walls 2 support rungs 3 substantially lower than roller rungs 10. However, in such an arrangement, the roller rungs 10, relatively fewer in number, are destined to shoulder all weight of the cable W laid on the cable bed 1, which will be too much a burden for such fragile conventional roller rungs 10.
In addition, such an arrangement is impractical conventionally, for it is such support rungs 3 that are to support cable weight. Besides holding side walls 2 firmly together, conventional support rungs 3 are provided to support as well as frictionally hold cables W laid on a cable bed 1 so that the cables W once laid on the cable bed 1 will not be displaced longitudinally or laterally.
It is impractical either to provide a cable bed 1 with conventional roller rungs 10 alone, for it is required to incorporate too many roller rungs 10, which will be much more costly than utilizing support rungs 3.
Accordingly, it is an object of the present invention to provide a roller-support rung arrangement, which can efficiently assist in reducing traction force from cable drawing and can stably hold a cable or cables once laid thereon.
It is another object of the present invention to provide a cable bed incorporating such a roller-support rung arrangement, which can provide an easy laying of a cable or cables and stably hold the cable or cables once laid thereon.
It is a further object of the present invention to provide a cable laying method using such a cable bed, which can provide an easy laying of a cable or cables.